As is known in the art, a color cathode ray tube generally is constructed with a glass envelope having a color phosphor screen or layer formed on the inner surface of a panel of the glass envelope. A color selecting electrode is located within the envelope opposing the phosphor screen. An electron beam is emitted from an electron gun located within a neck portion of the envelope, the electron beam being scanned by an electromagnetic deflecting device for impingement on a desired phosphor or phosphors of the phosphor screen.
In conventional color cathode ray tubes having two-dimensionally curved color selecting electrodes or shadow masks, the curvature of the mask and its thickness causes it to be structurally self-supporting. Another type of commercial shadow mask is tensed on a cylindrical support frame and is not self-supporting as is the two-dimensionally curved type. It is used in conjunction with a cylindrically configured phosphor screen. A new type of shadow mask tube has a perfectly flat faceplate and an associated perfectly flat shadow mask. The shadow mask is a very thin foil maintained at a tension of tens of thousands of pounds per square inch. The afore-described cylindrical and flat tension shadow mask configurations are prone to vibrations, as may be caused by external pulses, or by a speaker in an associated television receiver, for example. The resonant frequency of vibration of the mask will vary depending on the mechanical parameters of and tension in the mask. Any vibration of the mask will cause electron beam landings to be out of registry with their respectively associated phosphor elements, causing color impurities in the reproduced images.
Various means have been suggested for damping the resonant vibrations described above. For instance, in U.S. Pat. No. 3,638,063 to Tachikawa et al, dated Jan. 25, 1972, a damping wire or rod is stretched across grid elements of the tube. With such an arrangement, the grid elements are resiliently pressed by the damping rod and, therefore, are not likely to be caused to vibrate by external mechanical shocks or electron beam bombardment. In U.S. Pat. No. 4,504,764 to Sakamoto, dated Mar. 12, 1985, resonant vibrations are damped by making the resonant frequency of at least one aperture grid element of the color selecting aperture grill so as to be different from that of another grid element in the vicinity thereof. It should be noted that with such prior art systems, (1) the grids or grills are cylindrically curved, rather than being flat, and (2) the grill includes a number of parallel band-shaped grid elements. Therefore, with the system of Tachikawa et al, the damping rod can be held against the grid elements because of the curved nature of the cathode ray tube screen. In Sakamoto, the grid elements themselves can be selected of different resonant frequencies. Such solutions to the problem of resonant vibrations are not appropriate with color cathode ray tubes using apertured shadow masks which are flat and in high tension. A damping rod or wire cannot be held in engagement with a flat shadow mask.
More particularly, a tension shadow mask is a rectangular membrane suspended in a high vacuum within the cathode ray tube envelope under high mechanical tension. The shadow mask is flat and, therefore, is capable of vibrating in so-called "membrane modes," i.e., the two-dimensional equivalent of the vibrations of a stretched string. This type of vibration is defined by the fact that the restoring force due to stiffness is negligible compared to that due to tension. The most prominent membrane mode is the fundamental one, with maximum amplitude in the center of the shadow mask. Elsewhere, the amplitude is a sinusoidal function of position. It is readily apparent that prior art mask damping devices, such as damping wires stretched in engagement with a cylindrically curved grill, are ineffective for use with a flat tension shadow mask. This invention is directed to providing a solution to the problem of damping resonant vibrations in a flat tension shadow mask.